1. Field of the Invention
The present invention relates to a method of removing a retainer of a jet pump and also relates to a jet pump after removal of the retainer, the method being for removing the retainer of a jet pump provided in a reactor pressure vessel of a boiling water reactor to circulate a cooling water to a reactor core or for removing a bolt for fastening the retainer.
2. Related Art
Conventionally, a so-called jet pump system combining a recirculating pump disposed outside a reactor pressure vessel and jet pumps provided in the reactor pressure vessel is employed in a boiling water reactor so as to increase power density.
As a method of carrying out forced circulation of reactor coolant to a reactor core portion of the reactor pressure vessel as cooling water in the boiling water reactor, there are provided an outside recirculation system and an inside recirculation system. The outside recirculation system includes a plurality of jet pumps disposed in the reactor pressure vessel and a recirculating pump disposed outside the reactor pressure vessel. The jet pumps serve to form cooling water sent from the recirculating pump into jet streams and forcibly send the cooling water from a reactor core lower plenum below a reactor core portion to the reactor core portion while taking in the reactor water around the jet pumps to thereby carry out the forced recirculation of the reactor coolant in the reactor pressure vessel. The jet pump of the boiling water reactor employing this jet pump system will be described hereunder with reference to FIGS. 9 to 12.
FIG. 9 is a vertical cross-sectional view schematically showing a structure of the boiling water reactor. As shown in FIG. 9, reactor coolant 2 and a reactor core 3 are housed in a reactor pressure vessel 1. The reactor core 3 is formed of a plurality of fuel assemblies, control rods, and the like, not shown, and is housed in a core shroud 10.
The reactor coolant 2 circulates upward through the reactor core 3. At this time, the reactor coolant 2 constitutes a two-phase flow of water and steam as its temperature is increased by nuclear reaction heat of the reactor core 3. The coolant 2 in the state of the two-phase flow flows into a steam-water separator 4 disposed above the reactor core 3 and is separated into water and steam there. The steam is then introduced into a steam dryer 5 disposed above the steam-water separator 4 and becomes dry steam.
The dry steam is transferred to a steam turbine, not shown, through a main steam pipe 6 connected to the reactor pressure vessel 1 and is used for electric power generation. On the other hand, the separated water flows through a downcomer portion 7 between the reactor core 3 and the reactor pressure vessel 1 and flows down below the reactor core 3. Control rod guide tubes 8 are disposed below the reactor core 3 and control rods are inserted into or withdrawn from the reactor core 3 through the control rod guide tubes 8.
A control rod drive mechanism 9 is disposed under the control rod guide tubes 8 and controls insertion and withdrawal of the control rods into and from the reactor core 3. In the downcomer portion 7, a plurality of jet pumps 11 are arranged with a space uniformly in a circumferential direction.
On the other hand, a recirculating pump, not shown, is disposed outside the reactor pressure vessel 1. The recirculating pump, the jet pumps 11, and recirculation piping disposed therebetween constitutes a recirculation system. The recirculating pump feeds drive water to the jet pumps 11 and forced circulation of the coolant 2 into the reactor core is carried out by the action of the jet pumps 11.
FIG. 10 is an enlarged view of an essential portion of FIG. 9. As shown in FIG. 10, the jet pumps 11 have a riser pipe 12. The riser pipe 12 is fixed to the reactor pressure vessel 1 through a riser place 20 and introduces the coolant 2 fed from a recirculation inlet nozzle 13 of the recirculating pump into the reactor.
A pair of elbows 15 are connected to an upper portion of the riser pipe 12 through a transition piece 14. Inlet throats 17 are respectively connected to the elbows 15 through mixing nozzles 16. The paired inlet throats 17 are respectively connected to diffusers 18. The mixing nozzles 16 jet the coolant 2, the reactor water around the nozzles 16 is taken in at this time, and the jetted coolant 2 and the taken water are mixed in the inlet throats 17. The elbow 15, the mixing nozzle 16, and the inlet throat 17 are integral with each other so as to form an inlet mixer 51.
By the way, in the above structure, fluid oscillation occurs due to the flow sent in from the recirculating pump in cooling. In order to cope with the fluid oscillation, a lower end of the riser pipe 12 is welded to the recirculation inlet nozzle 13 and an upper end of the riser pipe 12 is fixed to the reactor pressure vessel 1 through the riser place 20 as described above. The diffusers 18 are fixed to a baffle plate 26 welded to the reactor pressure vessel 1.
Upper ends of the inlet throats 17 are mechanically connected to the transition piece 14 through the mixing nozzles 16 and the elbows 15, and the lower ends of the inlet throats 17 are inserted into upper ends of the diffusers 18. In this way, the riser pipe 12 and the inlet mixers 51 have such structures as to be able to sufficiently cope with the fluid oscillation.
Next, a structure above the mixing nozzles 16 will be described. On opposite sides of the transition piece 14, a pair of ear portions 21 are formed, respectively. These ear portions 21 protrude upward, and groove portions 22 are formed on inner sides of upper end portions of the ear portions 21. A pair of jet pump beams 23, each having a rectangular section increasing in size toward a longitudinal center portion, are fixed to the groove portions 22 with their opposite end portions fitted in the groove portions 22.
FIG. 11 is a side view showing a fitted state of the jet pump beam 23 and FIG. 12 is a plan view of FIG. 11. As shown in these drawings, a screw hole is formed in a vertical direction at the center of the jet pump beam 23. In the screw hole, a head bolt 28 is screwed. A hexagon head is formed at an upper end of the head bolt 28, and a hemispherical head is formed at a lower end. On the other hand, the elbow 15 is formed with a mount portion having a horizontal upper end surface and the mount portion is formed with a counterbored hole. In the counterbored hole, the hemispherical head of the head bolt 28 is fitted through a spherical washer.
The inlet mixer 51 is not secured to the reactor pressure vessel 1, and therefore, the inflow water pressure of the drive water fed through the riser pipe 12 acts on the upper end portion of the inlet mixer 51. Moreover, the reaction force to the jet water pressure of the drive water jetted from the mixing nozzle 16 into the diffuser 18 also acts upward. In order to resist such load, the head bolt 28 may be screwed into the jet pump beam 23.
Since the ear portions 21 are fixed in fixed positions, when the head bolt 28 is screwed down, the jet pump beam 23 is moved upward, and the opposite ends of the jet pump beam 23 come in contact with the upper wall surfaces of the groove portions 22. In this way, an upward load is received.
On the other hand, a downward load is applied to an upper end portion of the elbow 15 through the head bolt 28, and magnitude of the load is determined by a relationship with the upward load due to the reaction force and the like of the drive water. A keeper 39 is detachably fitted over the hexagon head of the head bolt 28. The keeper 39 is secured onto a plate 40 by means of spot welding. The plate is in a square shape and is fixed to an upper surface of the jet pump beam 23 by means of two bolts.
A retainer 41 is fixed, below the head bolt 28, to the elbow 15 by a retainer mounting bolt 42 so that the inlet mixer 51, the head bolt 28, and the jet pump beam 40 can be handled as an integral body in removing the inlet mixer 51.
Incidentally, as the reactor operates, high-frequency oscillation is applied to the retainer 41 due to vane passing pulsation of the recirculating pump for recirculating the reactor coolant. The retainer 41 may lose its retaining force due to initial looseness of the retainer mounting bolt 42 or permanent set of the retainer 41 during the operation to come in contact with members therearound and wear away due to the oscillation caused by the circulation of the coolant. If this wearing proceeds, the retainer 41 may fall down into the reactor to cause damage fatal to the operation of the rector.
Conventionally, there is proposed a method of replacing a part in which a retainer is deformed by using a jig operated from a side of a jet pump top portion (e.g., Japanese Patent Application Laid-open NO. 8-201566).
Further, the retainer 41 has a function of fixing the head bolt 28 to the elbow 15 in removing the inlet mixer 51 after starting the operation of the reactor. If the retainer 41 is not installed during the normal operation, no functional problem will be caused. On the contrary, if the retainer 41 and the retainer mounting bolt 42 are damaged after the starting of the operation of the reactor, they may constitute falling objects.
Since an area around the retainer 41 and the retainer mounting bolt 42 becomes an environment of high-dose radiation due to irradiation of nuclear fuel, it is difficult to directly remove the retainer 41 and the retainer mounting bolt 42. Therefore, in order to obtain sufficient shielding effect of water, the retainer 41 need to be removed through an underwater-remote control operation. However, conventionally, there is no known method of detachment under such an environment that sufficient shielding effect of water is obtainable.